Meanwhile, firefighters in south London have been tackling a grass fire in an area the size of about four football pitches on Mitcham Common.

Level three alerts are triggered when threshold temperatures have been reached for one day and the following night, and the forecast for the next day has a greater than 90% confidence level that the day threshold temperature will again be met.

The threshold temperature for south-west England and the West Midlands is 30C (86F), while for the South East and London it is 31C (87.8F) and 32C (89.6F) respectively.

Level two warnings are currently in place for the east of England, East Midlands and Yorkshire and the Humber. These alerts are triggered by a 60% or higher chance of the threshold temperature again being reached on the second day.

North-east and north-west England are under a level one warning - the minimum state of vigilance.

Scotland, Wales and Northern Ireland are not included in the alert system.

It is the UK's first prolonged heatwave since 2006, with six consecutive days of temperatures above 30C (86F).

'Take action'

Ambulance services across England have reported an increase in call-outs since the hot weather began.

In West Yorkshire, sunbathers took to the beach at the Gaddings Dam reservoir, near Todmorden
Teenagers jumped into the harbour in Carnlough, Country Antrim
Environment Agency workers rescued fish stranded in isolated pools in dried out parts of the River Teme, near Leintwardine, Herefordshire
Some sought respite from the heat in the Appearing Rooms fountain, by Danish artist Jeppe Hein, on London's Southbank
Fire crews have warned smokers that failing to dispose of cigarettes properly can lead to grass fires like this one on Mitcham Common, south London
For some the hot weather is a good excuse to buy ice creams
For others it's a hot wait for the royal baby outside St Mary's Hospital
But Her Majesty keeps her cool overlooking the crowds on the first day of the Ashes Test match at Lord's

The North East Ambulance Service NHS Foundation Trust said it would usually expect 700 calls a day at this time of year but was receiving up to 1,250.

A number of these were to beaches and rivers for heat-related illnesses and other health problems, it added.

London Ambulance Service NHS Trust said it was receiving about 5,500 calls day - up by a thousand on last year.

It said heat-related call-outs included those for breathing difficulties, fainting, palpitations and alcohol-related incidents.

The warm weather is set to continue into next week with temperatures in the high 20Cs at the weekend before rising to 30C (86F) and beyond next week - bringing with it an increased risk of thunderstorms.

Heat health watch levels

Public Health England (PHE) officials have advised people to stay cool, drink lots of cold fluids and keep an eye on those they know to be at risk.

Professor Virginia Murray, head of extreme events at PHE, said: "A level three watch means take action. For us, summer is wonderful... but the real concern is that we are not used to heat in this country. We are not aware of the risks. So PHE has a heatwave plan for England which was published for this summer."

She told BBC Radio 4's Today programme that the plan was particularly targeted at healthcare providers, though there was advice for individuals.

She said: "The most important advice is to stay cool yourself, drink plenty of cool drinks - but look out for others, take care of the most vulnerable, take care of those who are very young, the elderly, those with chronic illnesses who may be particularly vulnerable to the heat and really to protect those as far as we can and to make sure we reduce the health impacts on them and the possible worrying level of increased deaths."

Care home manager

Sally Colwill, who manages Chatsworth Care Home in Plymouth, says the hot weather makes her elderly residents very lethargic.

Speaking to BBC Radio 4's Today programme, she said: "They can't sit outside because they get breathless and too hot, but at the same time they don't want the doors and windows open because they believe there'll be a cold wind out there."

Ms Colwill said residents with dementia did not understand why they needed to change their habits.

"Just getting someone dressed in the morning… they are used to all their layers… Trying to explain to someone with dementia that they don't need to wear as much as they usually do is quite difficult," she said.

Though the home offered parasols and shady areas outside for the residents, the extreme heat "still affects their breathing, and they can get panicky", Ms Colwill said.

Prof Murray warned that many people would be reluctant to change their behaviour in response to the warm weather, which is why guidance for care homes was important.

Mike Tipton, professor of human physiology at the University of Portsmouth, told BBC Radio 5 live that signs that someone is suffering as a result of the heat should be evident.

He said: "As soon as people start to feel anything like light-headedness, feel that they're hyperventilating, if they've got a very rapid pulse, if they start to feel pins and needles in the fingers and around the limbs, slightly nauseous maybe, headaches, then really they need to stop any activity they're engaged in, move to the shade, sit down or lay down, preferably get help as well."

Jonathan McShane, from the Local Government Association, said councils are doing their bit to tend to those who may need extra assistance.

On BBC radio 5 live, he said: "Social workers, care workers, people who deliver 'meals on wheels', as part of their daily routine they're now doing an additional check to ensure that people are coping in the heat and they're staying hydrated and giving them some advice about how they can stay well and comfortable in the extreme weather."

Meanwhile, a four-week-old baby has been treated for severe sunburn at Queen Victoria Hospital in East Grinstead, West Sussex.

The hospital said it had seen 10 babies and children with severe sunburn in the past four weeks.